The present invention relates to an object detection apparatus of the photoelectric reflection type having a photoemitter and a photosensor for photoelectrically detecting an optical ray reflected from an object to determine whether an object is present or absent, and more specifically relates to a particular apparatus of the type for detecting an object which is entering an observation zone within a predetermined observation range along the optical axis. The present invention also relates to an object detection apparatus of the type for detecting when an object enters an observation zone by comparing intensity of the light reflected from the object with intensity of the light reflected from the background.
The known conventional object detection apparatus is constructed such that a photoemitter and photosensor are disposed adjacently on a common plane so as to detect an optical ray reflected from an object which crosses an optical axis perpendicular to the common plane.
However, the conventional object detection apparatus of the photoelectric reflection type has a drawback that a detection range is substantially infinite along the optical axis such that an irrelevant article would be detected incidentally as long as such article is not concealed by a background. There is a practical need for detecting only an object which is expected to enter into a given observation range, but the conventional object detection apparatus of the reflection type for general purpose cannot satisfy such need and would require an additional tool for limiting a travel distance of incident optical ray.
Conventionally, the known object detection apparatus is constructed such as to direct an incident light to a given observation zone and to compare the intensity of the light reflected from an object surface with a predetermined reference value to thereby detect the object.
However, such conventional detection apparatus uses an absolute reference value for judging as to whether an object is present or absent, thereby causing misjudgment that occurs due to various fluctuating factors. Such factors include a drift of the incident light intensity, variation of the reflectivity of the object surface, sensitivity change of the photosensor and influence of the background.
Particularly in the known object detection apparatus of the general purpose type, there is a drawback that the light reflected from the object cannot be discriminated from the light reflected from the background in terms of the intensity when using the absolute reference value due to variation of the background reflectivity and object distance.